Hydrogenation involves adding one or more hydrogen atoms to an unsaturated hydrocarbon (e.g., an olefin or aromatic compound). Hydrogenation can occur either as direct addition of hydrogen to the double bonds of unsaturated molecules, resulting in a saturated product, or it may cause the breaking of the bonds of organic compounds, with subsequent reaction of hydrogen with the molecular fragments. Examples of the first type (called addition hydrogenation) are the conversion of aromatics to cycloparaffins and the hydrogenation of unsaturated olefins, e.g., ethylene, to saturated olefins, e.g., ethane, by addition of hydrogen to the double bonds. Examples of the second type (called hydrogenolysis or hydrocracking) are the cracking of petroleum and hydrogenolysis of light hydrocarbon gases, e.g., ethane, to methane.
Some addition hydrogenation reactions, such as conversion of aromatics to cycloparaffins, are structure sensitive reactions. Structure sensitive reactions have reaction rates that are dependent upon the size of the active catalyst sites. Hydrogenolysis reactions can also be structure sensitive.
Hydrogenation is typically carried out in the presence of a catalyst comprised of a support, such as a natural clay, a synthetic metal oxide, or a crystalline molecular sieve such as zeolite, and a metal hydrogenation component.
Catalysts having a hydrogenation function are employed in a wide variety of organic compound conversion processes. An example of such a process involves the isomerization of xylene and dealkylation of ethylbenzene to benzene and ethane. This process is typically carried out by passing a para-xylene depleted C8 aromatic feed containing ethylbenzene over a catalyst comprised of a molecular sieve support, e.g., intermediate pore size molecular sieve, and a hydrogenation component, such as a Group VIII metal, e.g., platinum, or a Group VIIB metal, e.g., rhenium, to obtain ortho-, meta-, and para-xylene in a ratio approaching the equilibrium value while converting the ethylbenzene to benzene and ethane. The hydrogenation component is present in the catalyst to hydrogenate the ethylene formed in the dealkylation of ethylbenzene to ethane. An example of such a process is disclosed in U.S. Pat. No. 4,163,028.
It is important that the catalysts used in many organic compound conversion processes, such as xylenes isomerization/ethylbenzene conversion processes, have reduced hydrogenation activity. For example, if the catalyst used in xylenes isomerization/ethylbenzene dealkylation has hydrogenolysis activity that is too high, ethylene formed in the dealkylation of ethylbenzene to ethylene and benzene can be cracked to methane. This cracking reaction generates a large amount of heat, which can cause large exotherms inside the reactor, which can lead to damage of the catalyst, equipment, or both.
Also, in reactions involving aromatics conversion, catalysts having too high addition hydrogenation activity can result in aromatic ring saturation. Aromatic ring saturation results in aromatic molecules being converted to naphthene. These naphthenes can crack to light hydrocarbon gases when contacted with acid-based catalysts. Ring saturation can result in the loss of high value aromatics, e.g., xylenes.
One technique for passivating the catalyst (lowering the hydrogenation activity of the catalyst) involves treating the catalyst with a sulfur-containing compounds such as hydrogen sulfide gas or an organic sulfide compound. Such a technique is disclosed in U.S. Pat. No. 5,004,855.
When a sulfur treatment technique is used to reduce the hydrogenation activity of a catalyst, certain problems can arise. For instance, treatments using sulfur involve a toxic, corrosive, and pungent substance. Also, when the support material used in the catalyst is a molecular sieve having unidimensional ring pores, the sulfur treatment can block the pores of the molecular sieve, which usually results in reduced activity (and even deactivation) of the catalyst.
The present invention provides a process for reducing the hydrogenation activity of a catalyst that avoids treating the catalyst with sulfur compounds.